This invention relates to a radioactive waste sealing container which is used when low- to medium-level radioactive waste generated in atomic power plants, atomic power research installations, etc., is to be processed, disposed of, or stored.
When processing the low- to medium-level radioactive waste generated in atomic power installations, in general the inflammable material contained in the waste such as paper and fibers and burned and then sealed into a steel container (e.g. drum) coated on its inner and outer surfaces with a melamine-alkyd or epoxy resin paint for storage, while liquid or non-combustible solid contents are subjected to a concentration or compression treatment as they are, are then solidified by the use of cement or asphalt, and are thereafter sealed into a steel container of the type described above. However, the paint coating of the steel drums or containers coated with the melamine-alkyd or epoxy resin paint is likely to be damaged during such processes as the solidification treatment and sealing of the radioactive waste, or the transportation of the drum or container, and this damage results in the occurrence of corrosion of the steel base, accelerates the deterioration of the coating, and eventually promotes the corrosion of the steel base. The progress of the corrosion of the steel base due to the deterioration of the coating is markedly dependent upon the environmental conditions in which the drum or container is stored, but in view of the fact that atomic power installations are generally situated near the sea, external factors that can cause and promote corrosion such as brine particles and moisture are abundunt. The interior of the container is inevitably exposed to chemical action by radioactive waste containing chlorides and sulfates. For these reasons, if the coating is damaged for some reason or other, corrosion proceeds on both the inner and outer surfaces of the container so that leakage of radioactivity occurs, and the overall safety of atomic power is reduced.
To eliminate these problems, various proposals have been made in the past. For example, Japanese Patent Publication No. 957/1982 discloses a method which provides a foundation coating for the steel container by zinc plating or flame spraying with zinc, and then an organic paint containing a zinc phosphate is painted thereon. Japanese Patent Publication No. 958/1982 discloses a method in which a foundation coating is first provided over the steel container using a paint consisting of zinc as a principal component, or zinc plating or zinc flame spraying, and then a paint consisting principally of tar or asphalt is painted thereover. These prior art methods can improve the corrosion resistance to some extent, it is true, but they are not yet entirely satisfactory because the coatings swell as they are brought into contact with the contents of the container, such as chlorides and sulfates, for an extended period of time, and they have a rather low heat resistance.
The prevention of corrosion of the container has thus been one of the most essential requirements for the assurance of the complete safety of atomic power installations.
The container of the kind described above consists generally of a main container body and a lid, and after the lid has been placed over the main body, they are fixed together by fixing members obtained by applying a thin electroplating of zinc to a steel base, or by subjecting a steel base to a surface treatment such as zinc plating. Although the steel fixing members subjected to the surface treatment of a thin electroplating of zinc provides a corrosion resistance for a brief period of time, the thin zinc plating layer wears out gradually as the container is stored for a long period of time, and iron rust develops on the fixing members to deteriorate them earlier than the main container body, thereby breaking the seal between the main body and the lid of the sealing container.
To solve this problem, an attempt has been made conventionally to increase the thickness of the electroplating layers of the fixing members, but an extremely thick plating would reduce the adhesion of the plating to the steel base of the fixing members so that the plating surface will become non-uniform, and the corrosion-resistant metal coating will peel off from the plating surface. Morever, since there is an inherent limit to the thickness of an electroplating layer, this method is not always effective.
A surface treatment such as painting has been made over a steel base but this method is not entirely satisfactory, either, because the coating is likely to be damaged during the transportation of the radioactive waste sealing container, and eventually this causes the corrosion of the steel base immediately after the start of the storage of the container, accelerates the corrosion of the coating and eventurally promotes the corrosion of the steel base. Moreover, since oxygen concentration cells are formed in the spaces between the container lid and the fixing members, the cell action promotes the corrosion of the lid, causes a leakage of radioactivity, and thus deteriorates the safety of atomic power. The prevention of the corrosion of the fixing members has thus been another essential requirement for the assurance of the complete safety of atomic power installations.